1. Field of the Invention
The invention relates to an air-fuel ratio control apparatus of an internal combustion engine, which controls the air-fuel ratio of gas flowing into a catalyst arranged in an exhaust passage of the internal combustion engine, and a control method of that air-fuel ratio control apparatus.
2. Description of the Related Art
Japanese Patent Application Publication No. JP-A-2004-183585, for example, describes a widely known air-fuel ratio control apparatus which is provided with an air-fuel ratio sensor both upstream and downstream of a catalyst (a three-way catalyst) arranged in an exhaust passage of an internal combustion engine, and feedback controls the air-fuel ratio of the air-fuel mixture supplied to the internal combustion engine (and therefore the air-fuel ratio of the gas flowing into the catalyst) based on output values from the air-fuel ratio sensors. The apparatus described in that publication uses a so-called limiting current oxygen content sensor for the upstream side air-fuel ratio sensor and a so-called concentration cell oxygen content sensor for the downstream side air-fuel ratio sensor.
Air-fuel ratio sensors are generally known to have a characteristic in which their sensitivity changes, which in turn results in a change in output value, when the manner in which the gas contacts the detecting portion (hereinafter simply referred to as “manner of gas contact”) changes, even if the air-fuel ratio of the gas that contacts the detecting portion is the same.
Here, the manner in which gas that is discharged from the combustion chamber into an exhaust passage via an exhaust valve flows (such as the direction of the flow and the flow rate) changes according to the operating state of the engine. Accordingly, the manner in which gas contacts the detecting portion of the upstream side air-fuel ratio sensor arranged in the exhaust passage in a position relatively near the exhaust valve can also change according to the operating state of the engine. As a result, the output value of the upstream side air-fuel ratio sensor depends on the manner of gas contact and thus may change even if the air-fuel ratio of the gas contacting the detecting portion is the same.
In addition, an air-fuel ratio sensor is also generally known that has a characteristic in which the output value changes when the composition of gas contacting the detecting portion changes, even if the air-fuel ratio of that gas is the same. More specifically, there is a tendency for the rate of diffusion to increase in the detecting portion the smaller the molecular mass of the composition of the gas that contacts the detecting portion. Accordingly, for example, even if the air-fuel ratio of the gas contacting the detecting portion is the same, the output value of the air-fuel ratio sensor tends to shift to a richer value when the molecular mass in the gas contacting the detecting portion is small and the composition ratio of hydrogen which is a reducing agent is large.
Here, the composition of the gas that is discharged from the combustion chamber into the exhaust passage via the exhaust valve also changes according to the operating state of the engine. Accordingly, the composition of the gas contacting the detecting portion of the upstream side air-fuel ratio sensor arranged in the exhaust passage in a position relatively close to the exhaust valve can also change according to the operating state of the engine. As a result, the output value of the upstream side air-fuel ratio sensor may change depending on the composition of the gas, even if the air-fuel ratio of the gas contacting the detecting portion is the same.
As described above, the output value of the upstream side air-fuel ratio sensor may be different from a value corresponding to the true air-fuel ratio of the gas contacting the detecting portion depending on the manner of gas contact or the composition of the gas. Therefore, appropriate feedback control may not be performed on the air-fuel ratio if it is performed using the output value itself of the upstream side air-fuel ratio sensor.